Current transformer terminated by active load elements for providing phase-inverted signals

ABSTRACT

The secondary winding of a current transformer is terminated in a virtual short-circuit condition by a pair of active load elements, such as transresistance amplifiers, which provide, for example, a pair of phase-inverted output voltages proportional to current in the primary winding of the current transformer.

United States Patent 1191 323/87, 88,120,108-110; 324/ll0, 123, t 127;317/16; 328/155; 321/47 Milkovic June 4, 1974 CURRENT TRANSFORMER TERMINATED [56] References Cited BY ACTIVE LOAD ELEMENTS FOR UMTED STATES PATENTS PROVIDING PHASE'INVERTED S 2.981.888 4/196] White Jr. 324/110 x Inventor: Miran Milkovic, Scotia Kernick 6 ill. C I [73] Assignee: g g g g g Primary Examiner-Gerald Goldberg C enema Attorney, Agent, or Firm--Patrick D. Ward; Joseph T. [22] Filed: Mar. 30, 1973 Cohen; Jerome C. Squillaro [2]] Appl. No.: 3tt6,4ll [57] ABSTRACT The secondary winding of a current transformer is ter- [52 l 7 2 minated in a virtual short-circuit condition by a pair of I C 6 19 00 active load elements, such as transresistance amplifif i 50 g ers, which provide, for example, a pair of phase. 0 inverted output voltages proportional to current in the primary winding of the current transformer.

17 Claims, 2 Drawing Figures PATENTEDJUM 4 I974 3.8 1501 1 CURRENT TRANSFORMER TERMINATED BY ACTIVE LOAD ELEMENTS FOR PROVIDING PHASE-INVERTED SIGNALS CROSS-REFERENCES TO RELATED PATENT APPLICATIONS A related, now abandoned, US. Pat. application Ser. No. 262,643, earlier filed on 14 June I972 in behalf of M. Milkovic and titled CURRENT TRANSFORMER WITH ACTIVE LOAD TERMINATION, and a related U.S. continuation Pat. application Ser. No. 365,429, filed 3l May 1973, in behalf of the same M. Milkovic and having the same title as the aforesaid abandoned patent application, discloses, inter alia, a current transformer terminated in a virtual short circuit by an active load element, such as a transresistance amplifier, which also produces an output signal voltage proportional to current in the primary "winding of the current transformer.

Another related co-pending US. Pat. application Ser. No. 346,412 filed on Mar. 30, I973, concurrently herewith, in behalf of M. Milkovic and titled CUR- RENT TRANSFORMER WITH ACTIVE LOAD TER- MINATION FOR PROVIDING, INTER ALIA, PHASE ANGLE ALTERATION discloses, inter alia, a current transformer terminated in a virtual short circuit by a transresistance, or trans-impedance, amplifier which produces an output voltage signal proportional to primary current in the transformer; said amplifier includes means for shifting the phase angle between primary, or secondary, transformer current and said output voltage.

The entire right, title and interest in and to the inventions described in the aforesaid patent applications, as well as in and to the aforesaid patent applications, and

' the entire right, title and interest in and to the invention herein disclosed, as well as in and to the patent application of which this specification is a part, are assigned to the same assignee.

SUMMARY OF THE INVENTION The subject invention, pertains, in general, to terminating instrument current transformers in a virtual short circuit condition while enabling the developement of an output voltage signal which is proportional to current in the primary winding of the transformer; and, in particular, to terminating an instrument current transformer in a virtual short circuit condition while enabling the development of multiple output voltage signals, each of which is proportional to current in the primary winding of the current transformer as well as phase-displaced relative to the other voltage signals developed.

In accordance with an exemplary embodiment of the invention there is provided a current transformer having primary and secondary windings which are adapted for conducting primary and secondary current, respectively. A pair of active load elements in the form of operational amplifiers, functioning as transresistance amplifiers, is provided. Each operational amplifier has a relatively high gain and includes inverting and noninverting input terminals as well as an output terminal. Between the inverting and non-inverting input terminals of each amplifier a relatively low input impedance exists due to negative feedback action.

The ends of the secondary windings are connected to the inverting input terminals of different operational amplifiers and the non-inverting input terminals of the amplifiers are commonly connected so that the secondary winding is terminated in a virtual short circuit condition. A pair of feedback impedances (e.g., resistors) is provided. Each feedback impedance is connected in parallel with a different one of the operational amplifiers, between an inverting input terminal and output terminal thereof. The amplifiers produce a pair of output voltages at their output terminals in response to current in the secondary winding of the current transformer. These voltages appear between the output terminals of each amplifier and the commonly connected noninverting terminals of the amplifiers. The voltages are phase-displaced relative to eace other; e.g., out of phase.

Various objects, features and advantages of the invention are set forth hereinafter where specific illustrative embodiments of the invention are disclosed in detail with reference to the accompanying drawings.

DRAWINGS FIG. 1 is a schematic illustration of the invention showing an instrument current transformer terminated in a virtual short circuit condition by a pair of transresistance amplifiers.

FIG. 2 is another schematic illustration showing another embodinent of the invention where an instrument current transformer includes a center-tapped secondary winding, said secondary winding being termintated in a virtual short circuit condition by a pair of transresistance amplifiers.

DESCRIPTION OF PREFERRED EMBODIMENTS At FIG. 1 an instrument current transformer CT hav ing a primary winding of Np turns and a secondary winding of Ns turns is shown. The current transformer turns ratio is K Ns/Np. As indicated, the primary winding Np conducts primary current Ip and a secondary winding Ns conducts secondary current Is. Thus, K Ns/Np Ip/Is from well known relationships respecting transformer operation.

As shown at FIG. 1 two oppositely poled diodes D1 and D2 are connected in parallel with each other as well as in parallel with the secondary winding Ns. In the event that the secondary section of the current transformer should be disconnected at the terminals A and B the diode D1 and D2 serve to prevent the secondary winding Ns from being open circuited.

Also shown at FIG. 1 are a pair of operational amplifiers Al and A2 which are substantially identical, electrically. Each operational amplifier Al and A2 includes an inverting input terminal designated by a minus sign and a non-inverting input terminal designated by a plus sign The inverting input terminal of the operational amplifier Al is electrically connected to one end of the secondary winding Ns while the inverting input terminal of the oprational amplifier A2 is electrically connected to the other end of the secondary winding Ns. As shown at FIG. 1, the noninverting input terminals of both operational amplifiers Al and A2 are commonly connected to a signal or reference ground. Also, a pair of feedback resistors in the form of the resistance elements R1 and R2 are provided. One end of the resistance element R1 is connected to the output of the operational amplifier A1 while the other end of the same resistance element is connected to the inverting input terminal of operational amplifier A1 at a summing point S1. Similarly, one end of the resistance element R2 is connected'to the output of the operational amplifier A2 while the other end of the same resistance element is connected to the inverting input terminal of the operational amplifier A2 at a summing point S2. R1 and R2 may be ohmic resistance elements.

The opposite ends of the secondary winding of the transformer, at the summing points SI and S2, are at the same potential as the non-inverting input terminals of amplifiers Al and A2 due to feedback action of the transresistance amplifiers formed by operational amplifiers A1 and A2 and their associated feedback resistance elements R1 and R2. At the output terminals of each of the operational amplifiers A1 and A2 there are developed the output signal voltages VI and V2 which, with respect to the reference or signal ground (the noninverting input terminals of the operational amplifiers Al and A2), are 180 out of phase.

The current transformer CT shown in FIG. I has only one secondary winding Ns which is loaded with active load elements on both of the terminals A and B of the secondary winding. InFIG. 1 the combination of the operational amplifier Al and its feedback resistance elements R1 are connected as indicated to form a first transresistance amplifier or first active load element. Similarly, the operational amplifier A2 together with its feedback resistance element R2 forms a second transresistance amplifier or second active load element. Since the potential at the summing points S1 and S2 are substantially the same as at the non-inverting input terminals of A1 and A2 the secondary winding Ns of the current transformer CT, in effect, sees" a short circuit condition. Both operational amplifiers A1 and A2 are operated in their linear ranges.

The operational amplifiers Al and A2 shown in FIG. 1 (as well as those shown in FIG. 2) are high performance operational amplifiers pt A741 operational amplifiers manufactured by Fairchild Semiconductor, a division of Fairchild Camera and Instrument Corporation, 313 Fairchild Drive, Mountainview, California. These operational amplifiers are more fully disclosed, structurally and operationally, in the US. Pat. application Ser. No. 262,643 365,429, hereinbefore more fully identified. I

In the embodiment shown in FIG. 2 like elements have been designated with the same reference characters as employed in FIG. 1. As shown in FIG. 2 the operational amplifier A1 together with its feedback resistance element RI forms a first transresistance amplifier, or first active load element. Similarly, the operational amplifier A2 together with its feedback resistance R2 forms a second transresistance amplifier, or second active load element. The current transformer CTl includes a center-tapped secondary winding NS. Both of the transresistance amplifiers, hereinbefore identified, operate as Class A amplifiers connected in a push-pull arrangement with the current transformer CH. The summing points S1 and S2 are at virtually the same potential as the non-inverting input terminals of the amplifiers Al and A2. The output voltages V1 and V2, indicated at FIG. 2, are 180 out of phase.

The voltages V1 and V2 in both FIGS. 1 and 2 are proportional to the primary current 1p in the primary winding of the respective current transformers.

Although specific embodiments of the invnetion have been illustrated and described in detail to illustrate the invention, it is to be understood that the invention may be otherwise embodied without departing from the spirit and scope of the invention which is hereinafter set forth in the claims.

What is claimed is:

1. In combination: a current transformer having primary and secondary windings; and, a pair of transresistance amplifiers, each having inverting and noninverting input terminals as well as an output terminal, the inverting input terminals of said pair of amplifiers being connected to different ends of said secondary winding and the non-inverting input terminals of said pair of amplifiers being commonly connected, each amplifier having a relatively low input impedance, said inverting and non-inverting input terminals of said pair of transresistance amplifiers having substantially the same potential thereat.

2. The combination set forth in claim 1 wherein said secondary winding includes a center tap and wherein said non-inverting terminals are commonly connected to said center tap.

3. The combination set forth in claim 1 further comprising a pair of oppositely poled diodes connected in parallel with each other and with said secondary windmg.

4. The combination set forth in claim 1 wherein said pair of amplifiers are substantially identical, electrically.

5. The combination set forth in claim 2 wherein said pair of amplifiers are substantially identical, electrically.

6. In combination: a current transformer having primary and secondary windings; a pair of operational amplifiers, each having inverting and non-inverting input terminals as well as an output terminal, the inverting input terminals of said pair of amplifers being connected to different ends of said secondary winding and the non-inverting terminals of said pair of amplifiers being commonly connected; and, a pair of feedback I impedances, each being connected in parallel with a different one of said amplifiers between an inverting input and output terminal thereof, said inverting and non-inverting terminals of each amplifier having substantially the same potential thereat.

7. The combination set forth in claim 6 wherein said secondary winding includes a center tap and wherein said non-inverting terminals are commonly connected with said center tap.

8. The combination set forth in claim 6 further comprising a pair of oppositely poled diodes connected in parallel with each other and with said secondary windmg.

9. The combination set forth in claim 6 wherein said pair of amplifiers are substantially identical, electrically.

10. In combination: a current transformer having primary and secondary windings adapted for conducting primary and secondary current, respectively; a pair of operational amplifiers, each having inverting and noninverting input terminals, between which there is substantially no difference of potential, and an output terminal, the inverting input terminals of said pair of amplifiers being connected to different ends of said secondary winding and the non-inverting input terminals being commonly connected so that said secondary winding is terminated in a virtual short circuit; and, a pair of feedback impedances, each being connected in parallel with a different one of said amplifiers between 13. The combination set forth in claim 12 wherein said output voltages are out of phase.

14. The combination set forth in claim 11 wherein said pair of output voltages are phase-displaced relative to each other.

15. The combination set forth in claim 14 wherein said output voltages are 180 out of phase.

16. The combinatin set forth in claim 10 wherein said pair of amplifiers are substantially identical, electrically.

17. The combination set forth in claim 16 wherein said pair of amplifiers are operable in their linear ranges. 

1. In combination: a current transformer having primary and secondary windings; and, a pair of transresistance amplifiers, each having inverting and non-inverting input terminals as well as an output terminal, the inverting input terminals of said pair of amplifiers being connected to different ends of said secondary winding and the non-inverting input terminals of said pair of amplifiers being commonly connected, each amplifier having a relatively low input impedance, said inverting and non-inverting input terminals of said pair of transresistance amplifiers having substantially the same potential thereat.
 2. The combination set forth in claim 1 wherein said secoNdary winding includes a center tap and wherein said non-inverting terminals are commonly connected to said center tap.
 3. The combination set forth in claim 1 further comprising a pair of oppositely poled diodes connected in parallel with each other and with said secondary winding.
 4. The combination set forth in claim 1 wherein said pair of amplifiers are substantially identical, electrically.
 5. The combination set forth in claim 2 wherein said pair of amplifiers are substantially identical, electrically.
 6. In combination: a current transformer having primary and secondary windings; a pair of operational amplifiers, each having inverting and non-inverting input terminals as well as an output terminal, the inverting input terminals of said pair of amplifers being connected to different ends of said secondary winding and the non-inverting terminals of said pair of amplifiers being commonly connected; and, a pair of feedback impedances, each being connected in parallel with a different one of said amplifiers between an inverting input and output terminal thereof, said inverting and non-inverting terminals of each amplifier having substantially the same potential thereat.
 7. The combination set forth in claim 6 wherein said secondary winding includes a center tap and wherein said non-inverting terminals are commonly connected with said center tap.
 8. The combination set forth in claim 6 further comprising a pair of oppositely poled diodes connected in parallel with each other and with said secondary winding.
 9. The combination set forth in claim 6 wherein said pair of amplifiers are substantially identical, electrically.
 10. In combination: a current transformer having primary and secondary windings adapted for conducting primary and secondary current, respectively; a pair of operational amplifiers, each having inverting and non-inverting input terminals, between which there is substantially no difference of potential, and an output terminal, the inverting input terminals of said pair of amplifiers being connected to different ends of said secondary winding and the non-inverting input terminals being commonly connected so that said secondary winding is terminated in a virtual short circuit; and, a pair of feedback impedances, each being connected in parallel with a different one of said amplifiers between an inverting and output terminal thereof, said pair of amplifiers producing between the output terminals thereof and the commonly connected non-inverting terminals thereof a pair of output voltages in response to current in said secondary winding, said output voltages being proportional to current in the primary winding.
 11. The combination set forth in claim 10 wherein said secondary winding includes a center tap and wherein said commonly connected non-inverting terminals are connected to said center tap.
 12. The combination set forth in claim 10 wherein said pair of output voltages are phase-displaced relative to each other.
 13. The combination set forth in claim 12 wherein said output voltages are 180* out of phase.
 14. The combination set forth in claim 11 wherein said pair of output voltages are phase-displaced relative to each other.
 15. The combination set forth in claim 14 wherein said output voltages are 180* out of phase.
 16. The combinatin set forth in claim 10 wherein said pair of amplifiers are substantially identical, electrically.
 17. The combination set forth in claim 16 wherein said pair of amplifiers are operable in their linear ranges. 